A study of the autoxidation of hydroquinones and quinones by electron spin resonance spectroscopy

Abstract

The work reported in this thesis started from an investigation of the types of reactions occuring during the autoxidations of hydroquinones and quinones in alkaline solution. The free radical intermediates were identified by means of their E.S.R.spectra. In dilute alkali the primary radicals were observed, but in stronger alkali the spectra obtained corresponded to hydroxylated 'secondary radicals' which were characterised by producing identical spectra in the alkaline autoxidation of 1,2,4- Triacetoxybenzene derivatives. In some cases different isomeric radicals were obtained in different solvents.Added hydrogen peroxide had no apparent effect on the course of the reaction and the ease of formation of the secondary radioals seemed to depend on the concentration of the alkali and the presence of a fairly sterically unhindered site in the quinonoid ring. The adduct I is suggested as the intermediatein the formation of the secondary radical II, which is shown to be capable of reducing excess quinone back to the primary radical. When the quinonoid ring is free of bulky substituents, tertiary radicals are often observed under certain conditions, and these appear to arise from coupling through carbon at some stage of the reaction sequence. The coupling constants of the primary radicals containing a wide range of substituents ( X= alkyl, OMe, Cl etc. ) show that the semiquinone ring splittings appear to depend on the electron-releasing ability of X and on its steric bulk. The electron-releasing effect influences the spin densities ortho and para to the substituent, while spin densities meta to X are affected mainly by the steric interaction between X and the adjacent carbonyl group. The splittings in the secondary radicals also show these trends.The study of the hyperfine splittings in a wide range of Para-substituted Arylsemiquinones appears to show the effects of varying the electron-donating ability of the substituent exclusively. The splittings of semiquinone ring protons ortho to the substituent are decreased by more electron-releasing groups while those para are increased.. The meta proton splittings are fairly independent of the para-substituent in the aryl group.The observation of aryl group splittings in arylsemiquinones led on to a general study of the effects of changing the dihedralangle between the two rings. The aryl group splittings are found to vary in different ways for ortho, meta and para protons on increasing the dihedral angle by the addition of bulky ortho substituents. The variation of the splittings can be rationalised by considering the two possible mechanisms by which spin density can be transferred from the semiquinone ring to the aryl ring.The u-delocalisation mechanismC important at low values of the dihedral angle) and r-spin delocalisation (important as 0 approaches 90°), treated separately, account quite well for the variation of the splittings for protons and other groups (Me, F,) at the various positions in the aryl ring.